In recent years, with growing demand of higher function of electronics, high-density integration within electronic components and high-density mounting of electronic components have been developed. Hence, printed wiring boards capable of high-density mounting and so on used for the electronic components have been developed in miniaturization, high density and multilayering than ever before. Hence, a printed wiring board excellent in low-thermal expansion property, and having small warpage upon connecting parts by reflow is required in order to solve the problem of decrease in rigidity of the substrate itself due to thinner design, a printed wiring board excellent in heat resistance is required to solve the problem of increase in amount of heat generation of printed wiring board due to density growth, a printed wiring board excellent in desmearing property upon plating process, and capable of sufficiently ensuring electrical conductivity between an upper layer metal wiring and a lower layer metal wiring is required as printed wiring boards are multilayering, and further a printed wiring board having a simple production process is required to realize fast mass production.
A prepreg used for production of a printed wiring board is generally produced by dissolving a resin composition comprising a thermosetting resin such as an epoxy resin as a main component in a solvent to prepare a resin varnish, and impregnating a base material with the resin varnish followed by drying by heat. In order to improve the heat resistance, low-thermal expansion property, flame resistance, etc. of the printed wiring board, a prepreg using a resin varnish containing an inorganic filler is produced.
However, since the viscosity of the resin varnish containing the inorganic filler in a large amount is high, sufficient impregnation of the resin composition into a base material and uniform impregnation of the inorganic filler have been difficult. The viscosity of the resin varnish containing a large amount of inorganic filler may be decreased by using a liquid epoxy resin as the main component of the resin composition, however, a prepreg prepared by using the resin varnish has a problem that after removing the solvent by drying, the prepreg has high tackiness, and stickiness, and its handling is difficult.
Japanese Patent Application Laid-Open (JP-A) No. 2006-36916 discloses that a slurry composition, in which a spherical silica particle having an average particle diameter of 0.1 μm or more and 5 μm or less and a silica nanoparticle having an average particle diameter of 1 nm or more and 50 nm or less are dispersed in an organic solvent, has silica fillers in a closest packing state due to the difference of particle diameter between the spherical silica particle and the silica nanoparticle, and a varnish composition prepared by compounding an epoxy resin in the slurry composition contains the silica fillers uniformly and in high compounding amount. Furthermore, an insulation film and prepreg produced using the varnish composition are disclosed. However, the epoxy resin used in Examples of JP-A No. 2006-36916 is merely a liquid epoxy resin in a solventless state.